80 NON-MARINE FOSSIL MOLLUSCA. 



since the closing epochs of Mesozoic time. Fresh-water mollusca ap 

 pear to have been less subject tban marine mollusca to those cosmical 

 influences which, from age to age and from epoch to epoch of geological 

 time, progressively impressed the marine mollusca with their wonderful 

 diversity of form and structure. 



While so large a proportion of the types among the fossil fresh-water 

 and land mollusca are clearly recognized among those now living, a 

 large proportion, if not the greater part of the fossil brackish-water 

 types which are presented in this article are different from those of the 

 corresponding fauna now living. This is doubtless due to the fact 

 that the lines of succession of most of the brackish- water mollusca, the 

 remains of which we have had opportunity for study, were denizens of 

 the brackish-water Laramie Sea, and were ciit off by the final freshen- 

 ing of its waters, from which there was no escape to the coasts of the 

 open sea, where they might have had a congenial habitat, while their 

 contemporaries of the land and fluvatile waters survived. This fact 

 has been mentioned on previous pages in connection with the presenta- 

 tion of the several types referred to. It is, however, a remarkable fact 

 that at least five of the types that are recognized among the fauna of 

 the Laramie Group, namely, Bathyomphalus, Gerithidea, Pyrgulifera, 

 Melanopsis, and true Melania, have never been found among the living 

 fauna of North America, but are represented by living species in the 

 old world. But this and other questions pertaining to the Laramie 

 invertebrate fauna will be discussed in a monograph of that fauna now 

 in preparation . 



